1. Field of the Invention
This invention relates to electric power steering equipment in which an auxiliary steering force is generated by an electric motor.
2. Description of the Prior Art
Power-assisted steering equipment for helping a driver to steer an automobile is well-known. In conventional power steering equipment, an auxiliary steering force is generated in accordance with the steering by a driver and the force thus generated is transmitted to a steering gear. Most of the power steering equipment put to practical use at the present time is actuated by a hydraulic drive. The hydraulic drive of conventional power steering equipment is equipped with a control valve, hydraulic cylinder and related structure to generate an auxiliary steering force by moving oil within the cylinder in accordance with a steering operation.
One significant disadvantage of the above-described known hydraulic power steering systems, especially in light of the trend toward smaller cars, is that the control valve, hydraulic cylinder and related structure are large in size. There is also encountered a large pressure loss within the connecting lines. To prevent this pressure loss, the line must be bent with a small curvature. However, in the case of a vehicle not having a large mounting space in the engine room, such as a front drive vehicle, these constraints make it difficult to mount the power steering equipment. Also, in conventional hydraulic drive units, an effective seal must be used to ensure that oil does not leak. Additionally, handling of the equipment is difficult.
In order to avoid the above-identified problems, it has been proposed to use a motor as a driving device for power steering equipment. With the use of such motors, however, there is a danger that the motor may burn out due to poor insulation if temperature of the motor is allowed to exceed a predetermined temperature.
In a motor-driven power steering unit, the mounting position of the power steering equipment in a vehicle is very important. Namely, it is necessary for the vehicle to have enough space to mount both the motor, as a driving device of the power steering equipment, and a reducer. The reducer reduces rotating speed of the motor and transmits the driving force to the steering gear. If the reduction gear ratio of the reducer is decreased, the reducer is capable of being miniturized. For this purpose, it is necessary for the reducer to be connected to the input side of the steering gear, that is, to the output side of a manual steering torque transmission mechanism. The reason for this required arrangement is that the rotating speed of the motor is first reduced by the reducer to be transmitted to the output side of the manual steering torque transmission mechanism, and then the reduced rotating speed is further reduced by the steering gear. Therefore, the reduction gear ratio of the reducer itself may be kept small.
In a usual vehicle, three steering shafts are disposed between the manual steering gear and the steering wheel. The steering shafts are connected to one another with predetermined inclination by means of universal joints.
The first steering shaft is connected to the steering wheel and is positioned inside the vehicle passenger compartment, also referred to as the vehicular room. In the vehicular room there is enough space to mount the power steering equipment, but the equipment thus mounted may obstruct the driving operation by the driver.
The second steering shaft is disposed between a toe board partitioning the vehicular room and the engine compartment. In this area, there are disposed various mechanical elements and, therefore, it is difficult to provide enough space to mount power steering equipment. In addition, since the second and third steering shafts are connected to each other with inclination of a predetermined angle, if the constant steering torque from the power steering equipment is applied to this second steering shaft, the steering torque transmitted to the third steering shaft fluctuates in the form of sine-wave, as shown in FIG. 1, in accordance with the steering angle. As a result, this fluctuating steering torque is applied to the steering gear even when the steering torque from the power steering equipment is constant.
There is relatively enough space in the vicinity of the third steering shaft for power steering equipment. However, since this third shaft is disposed in the engine compartment, temperature of the power steering motor is liable to rise, with a potential burn-out of the motor, because of poor insulation. A large-sized motor is relatively free from the temperature problems, but is difficult to mount because of its large size.